The invention relates to a chisel for a facility for breaking open the solidified crust on an electrolytic cell, in particular on a cell for producing aluminum, and a method for using the chisel in practice.
In the manufacture of aluminum from aluminum oxide the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The aluminum which separates out at the cathode collects under the fluoride melt on the carbon floor of the cell; the surface of this liquid aluminum acts as the cathode. Dipping into the melt from above are anodes which, in the conventional reduction process, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the carbon anodes; this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The concentration of aluminum oxide decreases in the course of the process. At an Al.sub.2 O.sub.3 concentration of 1-2 wt.% the so-called anode effect occurs producing an increase in voltage from e.g. 4-4.5 V to 30 V and more. Then at the latest the crust must be broken open and the concentration of aluminum oxide increased by adding more alumina to the cell.
Under normal operating conditions the cell is fed with aluminum oxide regularly, even when no anode effect occurs. Also, whenever the anode effect occurs the crust must be broken open and the alumina concentration increased by the addition of more aluminum oxide, which is called servicing the cell.
For many years now servicing the cell includes breaking open the crust of solidified melt between the anodes and the side ledge of the cell, and then adding fresh aluminum oxide. This process which is still widely practiced today is finding increasing criticism because of the pollution of the air in the pot room and the air outside. In recent years therefore it has become increasingly necessary and obligatory to hood over or encapsulate the reduction cells and to treat the exhaust gases. It is however not possible to capture completely all the exhaust gases by hooding the cells if the cells are serviced in the classical manner between the anodes and the side ledge of the cells.
More recently therefore aluminum producers have been going over to servicing at the longitudinal axis of the cell. After breaking open the crust, the alumina is fed to the cell either locally and continuously according to the point feeder principle or discontinuously along the whole of the central axis of the cell. In both cases a storage bunker for alumina is provided above the cell. The same applies for the transverse cell feeding proposed recently in U.S. Pat. No. 4,172,018.
The breaking open of the solidified electrolyte is carried out with conventional, well known devices fitted with chisels which are rectangular or round in cross section.
The under part of the chisel which comes into immediate contact with the solidified electrolyte when breaking through the crust is, in the case of the known devices, e.g. vertical to the sidewalls, or is in the form of a cone or blunted cone on the face vertical to the sidewalls of the chisel. In U.S. Pat. application Ser. No. 184,480, filed of even date herewith, a chisel shape providing a stamping or shearing action is described.
When using permanently installed crust breaking facilities an opening of close fit for the chisel is created in the crust as a result of the repeated servicing at relatively short intervals and previous operation of the chisel, i.e. only a very small space exists between the chisel and the crust which is broken open. Depending on the shape of the crust breaker, in particular the chisel, there is a greater or lesser risk of the chisel becoming jammed in this opening in the crust.